1. Field of the Invention
This invention relates to physiological pressure sensing systems, and in particular to such a system utilizing a disposable pressure transducer having a cantilevered stain gage sensor and embodying a design which minimizes effects of thermally induced stresses.
2. Description of the Background Art
An improved flow control apparatus for use in fluid flow systems, such as for monitoring blood pressure in humans and other animals, is disclosed in U.S. Pat. No. 4,291,702 of James E. Cole et al., which patent is owned by the assignee hereof. As disclosed therein, the flow control apparatus is adapted for use with catheters, such as used in invasive blood pressure monitoring by insertion into a patient's artery or vein. In normal use in such systems, a source of a sterile isotonic solution is connected to the catheter and the solution is caused to flow through the catheter into the patient at a low flow rate sufficient to maintain the catheter open and unclogged as by blood constituents and the like at the inserted end of the catheter.
As further pointed out in the Cole et al. patent, the catheter system is firstly flushed of air before insertion of the catheter into the patient. To permit the system to be rapidly filled with the sterile solution which is used to flush the system of air, the flow control apparatus of said patent further is arranged to selectively permit a high flow rate of the solution from the source to the catheter.
In one embodiment of the apparatus disclosed in the Cole et al. patent, a flexible diaphragm is extended across an opening to the pressure chamber of the housing of the apparatus. Means are provided for connecting a fluid actuated transducer to the housing so that deflections of the diaphragm cause corresponding deflections of the transducer.
A problem has arisen in the use of such systems relative to the high expense and unreliability of the pressure transducers employed. Such transducers have been quite expensive in order to provide the necessary accuracy. As the transducers must be cleaned and sterilized from time to time, the useful life thereof has been relatively short because repeated cleaning and sterilization renders them inoperative. Such inoperativeness presents a particularly vexatious problem in that it normally is discovered at the critical time of setting up the system with the patient since testing of the apparatus is impractical immediately after cleaning and sterilization.
Further, the use of reusable pressure transducers may cause cross-contamination and spreading of infection where complete cleaning and sterilization are not effected.
In one known form of an improved physiological pressure sensing system disclosed in copending application Ser. No. 398,399 of Schaberg et al., filed July 14, 1982, now U.S. Pat. No. 4,545,389 and owned by the assignee hereof, the high cost and unreliability problems of the prior art systems are eliminated by the provision of an improved throwaway device including a fast-slow flow control means and a pressure transducer means in a single housing. This device comprises a low-cost, high-accuracy transducer effectively permitting throwaway use of the device so that the device may be used with a single patient only, thereby completely avoiding cross-contamination and infection.
The device further includes means for connecting the source of sterile isotonic solution thereto and means for connecting the patient catheter thereto, and eletrical connection means for connecting electrical output means of the pressure transducer to an external monitor and the like, the connecting means comprising portions of the throwaway device.
The means for controlling solution flow from the source, such as an elevated bag or container, further includes flow restrictor means which maintains the pressure chamber of the device substantially at the patient's blood pressure, notwithstanding the continual delivery of solution at a low rate therethrough to the catheter for maintaining the catheter unclogged, as discussed above.
The pressure transducer sensing means is electrically isolated from the fluid in the pressure chamber so as to effectively prevent damage to the pressure transducer as by application of high voltages and transient electrical pulses of 400 to 500 joules such as may be applied to the patient in effecting certain treatments. Alternatively, the electrical isolation of the transducer from the pressure chamber avoids transfer of electrical current from the external monitoring system and, thus, effectively isolates the patient from a short circuit in the external system.
Pressure relief means are provided for preventing high pressure conditions which may appear in the pressure chamber from damaging the pressure transducer.
The arrangement of the device as a throwaway device permits it to be packaged and stored in sterile condition until desired for use. At that time, the sterile sealed package is opened and the system filled with sterile isotonic solution to facilitate invasive connection to the patient. The device provides a complete interconnection system between the source of sterile solution, the electrical monitoring apparatus, and the patient, and, thus, completely eliminates the possibility of cross-contamination, as well as assuring that a reliable new system is available for immediate use.
The housing and other components of the system are formed from low-cost molded synthetic resins, such as polycarbonate, having high strength, chemical and electrical resistance, and dimensional stability.
The pressure transducer utilizes a strain gage comprising a diffused pattern on a cantilevered single crystal silicon beam providing the major spring restraint in the pressure transducer. A flexible cable of electrical conductors is connected to the strain gage by suitable fine wires.
The pressure transducer includes a force collector including a diaphragm, a frame, and a link for connecting the diaphragm to the deflectible beam. The beam is provided with strain gage means. Illustratively, the beam is formed of silicon, with a resistive strain gage grid fused thereinto. The beam is mounted to the frame by an isolator for reducing the effect on the strain gage means of stresses which may be induced in the frame. In particular, one end of the upper surface of the beam is affixed to the isolator and the other end of such surface is connected to the link with the lower surface of the beam being free.
Low-cost manufacture is effected by forming the three elements of the force collector as a one-piece unit, and by forming the beam as a wafer element having a thickness of approximately 0.01 inch.
The discussed throwaway pressure monitoring device is extremely simple and economical of construction while yet providing highly desirable features as discussed above.